The proposed research involves an attempt to accurately describe and define the in vitro differentiation of skeletal muscle. The appearance of muscle associated proteins (myosin, AChE and CK) will be followed in the morphologically identifiable postmitotic prefusion G1 myoblasts. Studies will also be made to determine the point, during the protracted prefusion G1, where these cells become incapable of re-entering the proliferative pool. The role of cell density and 'conditioned-medium' factors in stabilizing the rounded G1 prefusion cells will also be explored.